Magnesium base die casting alloys



Patented Feb. 6, 1934 UNITED STATES MAGNESIUM BASE DIE CASTING ALLOYS Leo B. Grant and Otis E. Grant, Midland, Mich., assignors to The Dow Chemical Company, Mid land, Mich., a corporation of Michigan No Drawing. Application July 10, 1933 Serial No. 679,713

3 Claims.

The present invention relates to improved magnesium base alloys containing aluminum and manganese. One object of our invention is to produce magnesium base alloys which contains both aluminum and manganese that are capable of being die-cast readily.

The magnesium base alloys containing from 8 to 15 per cent of aluminum and manganese in amount usually from 0.1 to about 0.5per cent have a good degree of resistance to corrosion and are among the strongest of the known alloys possessing the characteristic lightness and other valuable properties of the magnesium alloys, but they are difiicult to cast successfully in permanent die molds under pressure, as in die-casting. For example, it is extremely difiicult, at times impossible, to fill a permanent die mold of usual design, and when the cast metal solidifies in the mold cracks develop which render the casting unfit for use. The development of cooling cracks is particularly noticeable when the castings are ribbed on the outer edges and thereby are held rigidly in the mold. The only known procedures employed at present to minimize this crack formation have been to design a die mold so as to provide for as thick sections as possible and to avoid ribs, core pins, and the like, which tend to prevent uniform or regular shrinkage on cooling.

Inasmuch as it is not possible in all instances to modify the shape of. the casting to suit the characteristics of the alloy, we have attempted to find other metals which, when added to the aforementioned magnesium-aluminum-manganese alloys, would provide an alloy which tends to fill the mold'readily and to solidify without developing cracks. magnesium-aluminum-manganese alloys which contain from 8 to 15 per cent of aluminum and from 0.1 to 0.5 per cent of manganese are improved as respects their die-casting properties by the addition of small amounts of silicon, usually from 0.35 to 1.0 per cent being sufficient for the purpose.

The alloys which form the subject matter of our invention, then, contain the four metals consisting of magnesium, aluminum, manganese, and silicon. The aluminum content coming within the scope of our invention is between 8 and 15 per cent, that of manganese between 0.1 and 0.5 per cent, silicon between 0.35 and 1.0 per cent, the balance being magnesium.

The ease of die-casting the alloy increases slightly as the aluminum content is increased from the lower to the upper limit of the proportions specified, but the tensile strength appears We have discovered that the to reach a maximum at about 10 per cent. The aluminum content, therefore, may be chosen in accordance with the maximum die-casting quality desired in relation to the tensile strength.- From about 9 to 12 per cent of aluminum is satisfactory for most uses.

Manganese has a limited solubility in the alloys of this invention. What appears to be the saturating quantity of manganese varies from about 0.1 to 0.3 per cent as the aluminum content varies from 15 to 8 per cent, respectively, since aluminum reduces the solubility of manganese in the alloy. We have found that from 0.1 per cent up to about 0.15 or 0.2 per cent of manganese is a sufficient amount to employ in most instances although up to 0.5 per cent may be introduced if desired.

In testing the casting quality of our new alloys, we prepared a die mold in which a die-casting could be made that was 18.5" long by 6" wide and 0.090" thick throughout except for the edges which were 0.25" thick. Into this mold we attempted to cast a number of magnesium-aluminum-manganese alloys containing from 8 to 15 per cent aluminum and from 0.1 to 0.5 per cent manganese, the balance being magnesium, that were substantially free from silicon. We have found that such alloys did not readily take the mold impression and flow lines on the surface of the casting spoiled their appearance and utility. In addition to these defects the castings usually were cracked near the edges. Commercial casting free from cracks and surface fiow lines were readily obtained from the same mold when the composition of the alloy was changed by adding thereto from 0.35 to 1.0 per cent of silicon. The maximum improving effect of silicon on the die-casting quality of these alloys is developed when the silicon content is from 0.5 to 1.0 per cent which amount preferably is employed. The addition of silicon also reduces the casting temperature approximately 25 F. over the temperature necessary for the alloys without silicon, thus reducing the tendency for flaws to be produced from excessive oxidation.

After casting our new alloys are capable of improvement in mechanical properties by the well known heat treatment procedures provided for magnesium alloys which are capable of undergoing solid solution changes or precipitation changes on prolonged heating.

These alloys may be prepared in any convenient manner by employing any of the well known metallurgical methods of melting and alloying magnesium, and any combination of proportions 1 A magnesium base alloy consisting of aluminum in amount from 8 to 15 per cent, manganese from 0.5 to 0.1 per cent, silicon from 0.35 to 1.0 per cent, the balance being magnesium.

2. A magnesium base alloy consisting .of aluminum in amount from 8 to 12 per cent, manganese from 0.3 to 0.1 per cent, silicon from 0.35 to 1L0 per cent, the balance being magnesium.

3. A magnesium base alloy consisting of aluminum in amount from 9 to 12 per cent, manganese from. 0.2 to 0.1 per cent, silicon from 0.5 to

1Z0 per cent, the balance being-magnesium.

. .LEO B. GRANT,

OTIS E. GRANT. 

